If B is better than A, and C is better than B, it follows by the transitive property that C is better than A. And yet, this is not always the case. Every child knows the Rock-Paper-Scissors game – the epitome of non-transitivity in which there is no clear hierarchy between the three choices, although every two-way interaction has a clear winner: paper beats rock, scissors beats the paper and Rock beats the scissors.
Evolution can also be teeming with non-transitive interactions. While natural selection – the process by which organisms better adapted to their environment are more likely to survive and pass on their genes – can be observed over shorter time intervals, there is still debate as to whether the gains fitness builds up over long, evolving time scales. In other words, one would expect successive adaptive events (such as two-way Roche-Paper-Scissors interactions) to result in a cumulative increase in physical fitness, so the very latest generation is still fitter than the whole of his genealogy. the ancestors. However, this turns out not to be true in all cases.
The evolutionary process therefore includes what are called non-transitive interactions, sometimes producing organisms less fit than its ancestors. However, experimental demonstrations of such non-transitivity have been lacking.
Until now. A group of scientists at Lehigh University led by Gregory Lang, an associate professor in the Department of Biological Sciences, recently provided empirical evidence that evolution can be nontransitive. Lang and his team identify a nontransitive evolutionary sequence thanks to a yeast evolution experiment over 1000 generations. In the experiment, an evolved clone surpasses a recent ancestor but loses in direct competition with a distant ancestor.
Non-transitivity in this case is the result of multi-level selection involving adaptive changes both in the yeast nuclear genome and in the genome of an intracellular RNA virus. The results, which provide experimental evidence that the continued action of selection can give rise to less fit organisms compared to a distant ancestor, are described in an article published in eLife Journal today called “Adaptive Evolution of Nontransitive Physical Form in Yeast” (DOI: 10.7554 / eLife.62238).
This study confronts two common misconceptions about evolution, according to Lang. The first, he says, is that evolution is a linear “progress walk” where each organism along a line of descent is fitter than any that came before it.
Lang and his colleagues set out to determine how non-transitivity arose along a particular line of genealogical descent. In their 1000-generation yeast experiment, the non-transitivity is due to the adaptation of the yeast nuclear genome combined with the progressive deterioration of an intracellular virus. Initially, the population produced a toxin encoded by a virus and was immune to the toxin. As the population adapted, it corrected beneficial nuclear mutations as well as mutations within the intracellular viral population that resulted in loss of toxin production. Over time, the most beneficial nuclear mutations correct themselves, and selection in the viral population resulted in loss of immunity to the toxin, as the toxin was no longer produced. When put in competition with its distant ancestor, the evolved population of 1000 generations lost because of the toxin produced by the ancestor.
“Another misconception is that there is only one locus of selection,” Lang explains. “Multi-level selection – as the name suggests – indicates that selection can act simultaneously on multiple levels of biological organization.”
In the context of this experiment, multi-level selection was common, Lang says. “Selection acts on many levels of biological organization, from genes in a cell to individuals in a population. Selection at one level can impact physical condition at another.
“In fact, when we extended our study of the evolution of the host virus genome to other populations, we found that almost half of the 140 or so populations we studied underwent multi-level selection, fixing adaptive mutations in nuclear and viral genomes, “he adds.
“Evolutionary laboratory experiments have proven to be very effective in studying the principles of evolution, but this work is the first to document a non-transitive interaction and provide a mechanistic explanation,” said co-author Sean W Buskirk, an assistant professor at West Chester University who collaborated on research when a postdoctoral student in Lang’s lab. “Ultimately, the presence of a virus in the ancestor has a huge impact on how evolved yeast populations compete and interact with one another.”
The work of co-author Alecia B. Rokes, then a biology student at Lehigh, focused on the competition of two intracellular viruses inside yeast cells in what she calls her own “club. fight against viruses “.
“I worked on two competing viruses within yeast cells to see if one or the other viral variant had an advantage over the other, resulting in a higher frequency and one virus supplanting the other,” explains Rokes, now a graduate student in microbiology at the University of Pittsburgh. “It was amazing to be part of the process of elimination, persistence and sheer curiosity to understand what was really going on in these populations.”
By showing that non-transitive interactions can occur along a line of genealogical succession, the team’s work has broad implications for the understanding of evolutionary processes by the scientific community.
“This resolves what evolutionary biologist Stephen Jay Gould has called the ‘first level paradox’, which is the inability to identify broad patterns of progress over long evolutionary time scales, despite clear evidence of a selection acting over successive short intervals of time, ”Lang says. “Further, it calls into question the existence of true fitness maxima and, more broadly, implies that directionality and evolutionary progress may be illusory.”